$(function(){
	$("body").append("<div id='main_bg'/>");
	$("#main_bg").append("<img src='../image/login.jpg' id='bigpic'>");
	cover();
	$(window).resize(function(){
		cover();
	});

}); 

function cover(){
	var win_width = $(window).width();
	var win_height = $(window).height();
	$("#bigpic").attr({width:win_width,height:win_height});
}